Loans and allowance for credit losses - Allowance for Credit Losses by Class of Financing Receivable Disaggregated by Measurement Methodology (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	$ 1,223	$ 2,039
Collectively evaluated for credit loss/impairment	135,497	149,405
Purchased credit deteriorated	21,234	18,945
Less: allowance for credit losses	157,954	170,389	$ 89,141	$ 31,139
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	354	373
Collectively evaluated for credit loss/impairment	13,609	13,493
Purchased credit deteriorated	680	882
Less: allowance for credit losses	14,643	14,748	10,881	4,805
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	96	95
Collectively evaluated for credit loss/impairment	36,013	54,065
Purchased credit deteriorated	2,513	4,317
Less: allowance for credit losses	38,622	58,477	22,842	10,194
Residential real estate | 1-to-4 family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	0	0
Collectively evaluated for credit loss/impairment	17,787	17,206
Purchased credit deteriorated	1,785	2,014
Less: allowance for credit losses	19,572	19,220	13,006	3,112
Residential real estate | Residential line of credit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	8	9
Collectively evaluated for credit loss/impairment	8,872	10,031
Purchased credit deteriorated	388	494
Less: allowance for credit losses	9,268	10,534	6,213	752
Residential real estate | Multi-family mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	0	0
Collectively evaluated for credit loss/impairment	11,103	6,326
Purchased credit deteriorated	554	848
Less: allowance for credit losses	11,657	7,174	2,328	544
Commercial real estate | Owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	70	30
Collectively evaluated for credit loss/impairment	2,952	4,062
Purchased credit deteriorated	587	757
Less: allowance for credit losses	3,609	4,849	9,047	4,109
Commercial real estate | Non-owner occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	694	1,531
Collectively evaluated for credit loss/impairment	35,414	33,706
Purchased credit deteriorated	14,071	8,910
Less: allowance for credit losses	50,179	44,147	18,005	4,621
Consumer and other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Individually evaluated for credit loss/impairment	1	1
Collectively evaluated for credit loss/impairment	9,747	10,516
Purchased credit deteriorated	656	723
Less: allowance for credit losses	$ 10,404	$ 11,240	$ 6,819	$ 3,002